When It Comes to Romantic Gift-Giving, Humans Aren’t Alone. Snails, Birds and Flies All Woo Each Other With Presents

Every year, Americans spend about $25 billion on presents for Valentine’s Day. When it comes time to propose, a sweetheart will spend an average of about $6,000 on an engagement ring. But such major gifts aren’t just confined to humans. Many species have found that romance often works better when it comes with presents.

Scientists have found that even some of the creepiest, crawliest creatures on the planet take part in amorous gift-giving. Male dance flies, katydids and certain spiders offer a free meal to the objects of their affection; snails include a shot of fortifying nutrients with their sperm. Apart from convincing prospective mates to share a romantic moment, such offerings can strengthen the female after mating and sometimes even lessen her likelihood of mating with others.

When it comes to non-human animals, though, think more of edible gifts than metal rings. “It’s like giving her a box of chocolates,” said Joris Koene, an ecologist at Vrije University Amsterdam who studies snail reproduction. Koene said nuptial gift-giving in wildlife usually applies to species with separate male and female sexes, in which males contribute a gift of energy or specific substances that the female can then invest in her eggs.

But researchers have also untangled why species that fall outside of heteronormative partnerships have developed different gift-giving traditions—or none at all.

Koene co-wrote a study in 2017 on snails that sought to understand what sort of gifting happens when species mate outside a male-female dichotomy. To figure out what kind of resource exchange was going on, Koene and his co-authors monitored the amount of nutrients like carbon and nitrogen transferred between two different species of hermaphroditic snails.

The species he focused on were the freshwater Lymnaea stagnalis, or the great pond snail, and the terrestrial Cornu aspersum, or the garden snail. The pond snail can choose the role of giver or taker in a relationship. By contrast, for the garden snail to successfully mate, both mollusks have to insert their reproductive organs into each other at the same time. “The penises have to be inserted into each other in order to mate. Otherwise nothing happens,” Koene explained.

This behavior made Koene and his colleagues wonder whether the nuptial gift concept even applied in this case. After all, if both sides exchange gifts along with their sperm, isn’t it a zero-sum game? To find out, researchers watched the garden snails have sex, then callously pulled them apart past the point of no return, but just before each of them had ejaculated. They then collected the resulting substance that the snails excreted.

Analyzing the contents, they found that the snails did indeed exchange some nutrients. But the mutual ejaculation erased any benefit that either individual might have received from the exchange. In fact, when accounting for the energy expended in the exchange, both individuals end up losing out in a strict economy of energy because of the effort spent getting busy.

In the case of the garden snails, researchers didn’t have to hurt or kill the animals to better understand their behavior. “We just traumatize them a little bit,” Koene said, adding that it was for all “for the sake of science.” But the great pond snails weren’t quite so lucky. Koene says they had to kill the animals to collect the substance.

While they found that the pond snails likely do contribute some nutrients to their partners, it wasn’t nearly enough to actually help in the production of eggs. “You would need about 20 or 30 ejaculates to have a proper contribution to one egg clutch,” Koene said. “For both of the [snail species] we tested, if you add things up it doesn’t really benefit them that much.”

In that way, the ejaculated nutrients really are like a box of chocolates: They may provide a nice momentary boost, but they don’t seem to help with child-rearing.

For the garden snails, nutrition isn’t the only gift they present each other during mating. The terrestrial snails also “give” each other a love dart—a slurry of mucus gland products that they shoot through their mate’s body wall. The darts contain hormones that make it more likely that the recipient will bear offspring than digest the gift of sperm.

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In response to the 2017 research into the nutritional value of simultaneous giving in hermaphroditic snails, Karim Vahed, an entomologist then at the University of Derby in England who published several papers on nuptial gifts, said that the study shows how the exchange is much more complicated when we account for the fact that some species don’t necessarily operate under a binary male-female relationship.

“It’s the first paper that I’m aware of that actually drills down into that question more deeply by actually looking at what quantity of substances is exchanged,” he said at the time.

Vahed added that the snail situation stands in contrast to the huge nuptial gifts that some insects donate to their partners. Male katydids, for example, produce a spermatophore during mating that can be up to 30 percent of their body mass, which contains both ejaculate and a mass of edible jelly. The female eats the jelly while the sperm enters her body, and then consumes any leftover sperm for added sustenance.

By giving a large gift to the female, who isn’t monogamous, the male ensures that she will spend some of her precious time consuming it and make it less likely she finds another mate. His ejaculate confers another reproductive benefit as well: It contains substances referred to by researchers as “ejaculate allohormones,” which switch off female sexual receptivity.

“The males could actually be subtly manipulating female behavior by prolonging ejaculate transfers,” Vahed said.

Other species also have a hunger for sex—and the nuptial gifts that come with it.

Dance flies in the U.S. Northeast and Canada are peculiar in the animal kingdom in that the females of the species are more ornamental than the males. During dawn and dusk, the females will take wing, sucking in huge quantities of air to inflate their bright orange abdomens. The inflated abdomens swell to disks about the third of a size of a penny, ensuring that the mosquito-sized males will see them silhouetted against the horizon.

“They’re like little insect peacock tails,” said biologist Darryl Gwynne in 2017, prior to his retirement from the University of Toronto Mississauga. “When they really get going, there will be thousands of them.” And like male peacocks, these insects sacrifice physical fitness for sex appeal: Their ornamented, scaled legs and puffy bodies makes them clumsy, so they are more likely to blunder into spiderwebs than the males of the species.

The females’ seemingly counterproductive behavior is motivated by a powerful, instinctive urge: the munchies. Male flies that see the puffy females will catch nutritious prey or pick up inedible tokens like leaves or stones and present them to potential mates. The meals, as it turns out, comprise the female’s only source of sustenance. “With the [female] dance flies, it’s hunger that drives them,” Gwynne said.

Hungry female dance flies will mate with as many males as they can. This satisfies their appetite, as well as ensures a healthy genetic mix among their offspring. But Luc Bussière, a researcher of selection and evolution formerly at the University of Stirling in Scotland, now at the University of Gothenburg in Sweden, who has worked with Gwynne on dance flies, said in 2017 that it’s in the best interests of the males to mate with a female who isn’t as flashy. That way, he’s more likely to be her one and only.

Or so one might think. Having a large abdomen, said Bussière, might indicate to a male that a female is in peak mating form. “You can understand why males would like to mate with really fat females,” he said. “First of all, she’s probably got more eggs. And more importantly, those eggs are probably nearly ready to be laid.” But seemingly large and fertile females could a trick of the eye, deceiving males with their inflated, shiny bodies.

Male gift-givers can be tricksters, too. Nursery web spiders catch insects and wrap them in webbing in an effort to woo their female consorts and avoid getting eaten themselves. But males who aren’t as good at hunting down bugs will often give the female a fake meal instead. “In some cases the males actually cheat by transferring something inedible,” Bussière says. The fact that they gift-wrap it allows them to hide inedible items inside.

Sometimes, male gift-givers can be quite gory in their delivery. Male great gray shrikes, a thrush-sized bird that lives across Europe, northern Africa and Asia, attract mates by offering them food. But the males don’t just set the gift down gently—they impale their prey on thorns and sharp sprigs in areas the females pass by often. The edible presents are rodents, lizards, large insects or even other birds. For the female, the larger the gift, the better.

Great gray shrikes are socially monogamous birds, but they do step out occasionally. A study from 2005 found that when they cheat, they often put in more work for their other lover. Observations of 22 males that successfully gave gifts to both their mate and another female revealed that the offering to the non-mate required an average of four times more energy. The non-mate gifts were far more likely to be hard-to-hunt rodents or other birds, while the female mate usually just got insects­—and sometimes no gift at all.

Not all birds resort to violence as part of their romantic gifts. Several species of penguins present their partners with pebbles during the mating season. Gentoo penguins are known to carefully pick out the smoothest pebbles they can find, while Adélie penguins sometimes sneakily steal stones from their neighbors nest to expedite the process. A gift of a pebble is as practical as it is cute­­—the penguins use the rocks to construct their nests. Without any trees or other plant material in the harsh Antarctic climates, pebble nests elevate eggs and hatchlings above the cold ground.

Bussière said that Koene’s work with hermaphroditic snails helps reveal the biases that human researchers exhibit when studying animals that don’t have separate sexes or operate in monogamous male-female partnerships.

“The male reproductive strategy of giving nuptial gifts cannot be generalized to any sexually reproducing animal,” said Monica Lodi, the lead author of the study with Koene. “If we don’t step outside that general habit, we draw incorrect conclusions about the prevalence of these behaviors or these ways of life.”

Joshua Rapp Learn | | READ MORE

Joshua Rapp Learn is a D.C.-based journalist who writes about science, culture and the environment. He has crossed the Sahara Desert, floated down the Amazon River and explored in more than 50 countries.

Rudy Molinek | READ MORE

Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.